Stimulation Waveforms with High- and Low-Frequency Aspects in an Implantable Stimulator Device

ABSTRACT

Waveforms for a stimulator device, and methods and circuitry for generating them, are disclosed having high- and low-frequency aspects. The waveforms comprise a sequence of pulses issued at a low frequency which each pulse comprising first and second charge-balanced phases. One or both of the phases comprises a plurality a monophasic sub-phase pulses issued at a high frequency in which the sub-phase pulses are separated by gaps. The current during the gaps in a phase can be zero, or can comprise a non-zero current of the same polarity as the sub-phase pulses issued during that phase. The disclosed waveforms provide benefits of high frequency stimulation such as the promotion of paresthesia free, sub-threshold stimulation, but without drawbacks inherent in using high-frequency biphasic pulses.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/393,606, filed Apr. 24, 2019, which is a non-provisional applicationof U.S. Provisional Patent Application Ser. No. 62/670,551, filed May11, 2018, which is incorporated herein by reference, and to whichpriority is hereby claimed.

FIELD OF THE INVENTION

This application relates to Implantable Medical Devices (IMDs), and morespecifically to circuitry and methods to create high- and low-frequencymultiplexed pulses in an implantable stimulator device.

INTRODUCTION

Implantable neurostimulator devices are devices that generate anddeliver electrical stimuli to body nerves and tissues for the therapy ofvarious biological disorders, such as pacemakers to treat cardiacarrhythmia, defibrillators to treat cardiac fibrillation, cochlearstimulators to treat deafness, retinal stimulators to treat blindness,muscle stimulators to produce coordinated limb movement, spinal cordstimulators to treat chronic pain, cortical and deep brain stimulatorsto treat motor and psychological disorders, and other neural stimulatorsto treat urinary incontinence, sleep apnea, shoulder subluxation, etc.The description that follows will generally focus on the use of theinvention within a Spinal Cord Stimulation (SCS) system, such as thatdisclosed in U.S. Pat. No. 6,516,227. However, the present invention mayfind applicability with any implantable neurostimulator device system.

An SCS system typically includes an Implantable Pulse Generator (IPG) 10shown in FIG. 1. The IPG 10 includes a biocompatible device case 12 thatholds the circuitry and a battery 14 for providing power for the IPG tofunction. The IPG 10 is coupled to tissue-stimulating electrodes 16 viaone or more electrode leads that form an electrode array 17. Forexample, one or more percutaneous leads 15 can be used havingring-shaped or split-ring electrodes 16 carried on a flexible body 18.In another example, a paddle lead 19 provides electrodes 16 positionedon one of its generally flat surfaces. Lead wires 20 within the leadsare coupled to the electrodes 16 and to proximal contacts 21 insertableinto lead connectors 22 fixed in a header 23 on the IPG 10, which headercan comprise an epoxy for example. Once inserted, the proximal contacts21 connect to header contacts 24 within the lead connectors 22, whichare in turn coupled by feedthrough pins 25 through a case feedthrough 26to stimulation circuitry 28 within the case 12, which stimulationcircuitry 28 is described below.

In the illustrated IPG 10, there are thirty-two electrodes (E1-E32),split between four percutaneous leads 15, or contained on a singlepaddle lead 19, and thus the header 23 may include a 2×2 array ofeight-electrode lead connectors 22. However, the type and number ofleads, and the number of electrodes, in an IPG is application specificand therefore can vary. The conductive case 12 can also comprise anelectrode (Ec). In a SCS application, the electrode lead(s) aretypically implanted in the spinal column proximate to the dura in apatient's spinal cord, preferably spanning left and right of thepatient's spinal column. The proximal contacts 21 are tunneled throughthe patient's tissue to a distant location such as the buttocks wherethe IPG case 12 is implanted, at which point they are coupled to thelead connectors 22. In other IPG examples designed for implantationdirectly at a site requiring stimulation, the IPG can be lead-less,having electrodes 16 instead appearing on the body of the IPG 10. TheIPG lead(s) can be integrated with and permanently connected to the IPG10 in other solutions. The goal of SCS therapy is to provide electricalstimulation from the electrodes 16 to alleviate a patient's symptoms,such as chronic back pain.

IPG 10 can include an antenna 27 a allowing it to communicatebi-directionally with a number of external devices discussedsubsequently. Antenna 27 a as shown comprises a conductive coil withinthe case 12, although the coil antenna 27 a can also appear in theheader 23. When antenna 27 a is configured as a coil, communication withexternal devices preferably occurs using near-field magnetic induction.IPG 10 may also include a Radio-Frequency (RF) antenna 27 b. In FIG. 1,RF antenna 27 b is shown within the header 23, but it may also be withinthe case 12. RF antenna 27 b may comprise a patch, slot, or wire, andmay operate as a monopole or dipole. RF antenna 27 b preferablycommunicates using far-field electromagnetic waves, and may operate inaccordance with any number of known RF communication standards, such asBluetooth, Zigbee, WiFi, MICS, and the like.

Stimulation in IPG 10 is typically provided by pulses each of which mayinclude a number of phases such as 30 a and 30 b, as shown in theexample of FIG. 2A. Stimulation parameters typically include amplitude(current I, although a voltage amplitude V can also be used); frequency(f); pulse width (PW) of the pulses or of its individual phases such as30 a and 30 b; the electrodes 16 selected to provide the stimulation;and the polarity of such selected electrodes, i.e., whether they act asanodes that source current to the tissue or cathodes that sink currentfrom the tissue. These and possibly other stimulation parameters takentogether comprise a stimulation program that the stimulation circuitry28 in the IPG 10 can execute to provide therapeutic stimulation to apatient.

In the example of FIG. 2A, electrode E1 has been selected as an anode(during its first phase 30 a), and thus provides pulses which source apositive current of amplitude +I to the tissue. Electrode E2 has beenselected as a cathode (again during first phase 30 a), and thus providespulses which sink a corresponding negative current of amplitude −I fromthe tissue. This is an example of bipolar stimulation, in which only twolead-based electrodes are used to provide stimulation to the tissue (oneanode, one cathode). However, more than one electrode may be selected toact as an anode at a given time, and more than one electrode may beselected to act as a cathode at a given time. Note that at any time thecurrent sourced to the tissue (e.g., +I at E1 during phase 30 a) equalsthe current sunk from the tissue (e.g., −I at E2 during phase 30 a) toensure that the net current injected into the tissue is zero.

IPG 10 as mentioned includes stimulation circuitry 28 to form prescribedstimulation at a patient's tissue. FIG. 3 shows an example ofstimulation circuitry 28, which includes one or more current sources 40_(i) and one or more current sinks 42 _(i). The sources and sinks 40_(i) and 42 _(i) can comprise Digital-to-Analog converters (DACs), andmay be referred to as PDACs 40 _(i) and NDACs 42 _(i) in accordance withthe Positive (sourced, anodic) and Negative (sunk, cathodic) currentsthey respectively issue. In the example shown, a NDAC/PDAC 40 _(i)/42_(i) pair is dedicated (hardwired) to a particular electrode node ei 39.Each electrode node ei 39 is connected to an electrode Ei 16 via aDC-blocking capacitor Ci 38, for the reasons explained below. Thestimulation circuitry 28 in this example also supports selection of theconductive case 12 as an electrode (Ec 12), which case electrode istypically selected for monopolar stimulation. In some designs, the caseelectrode Ec 12 may not have a DC-blocking capacitor 38, and thereforenot all potential electrode nodes selected for stimulation may have aDC-blocking capacitor. PDACs 40 _(i) and NDACs 42 _(i) can also comprisevoltage sources.

Proper control of the PDACs 40 _(i) and NDACs 42 _(i) allows any of theelectrodes 16 to act as anodes or cathodes to create a current through apatient's tissue, R, hopefully with good therapeutic effect. In theexample shown, electrode E1 has been selected as an anode electrode tosource current to the tissue R and E2 as a cathode electrode to sinkcurrent from the tissue R. Thus PDAC 40 ₁ and NDAC 42 ₂ are activatedand digitally programmed to produce the desired current, I, with thecorrect timing (e.g., in accordance with the prescribed frequency F andpulse widths PWa and PWb). Power for the stimulation circuitry 28 isprovided by a compliance voltage VH, as described in further detail inU.S. Patent Application Publication 2013/0289665. More than one anodeelectrode and more than one cathode electrode may be selected at onetime, and thus current can flow through the tissue R between two or moreof the electrodes 16.

Other stimulation circuitries 28 can also be used in the IPG 10. In anexample not shown, a switching matrix can intervene between the one ormore PDACs 40 _(i) and the electrode nodes ei 39, and between the one ormore NDACs 42 _(i) and the electrode nodes. Switching matrices allowsone or more of the PDACs or one or more of the NDACs to be connected toone or more electrode nodes at a given time. Various examples ofstimulation circuitries can be found in U.S. Pat. Nos. 6,181,969,8,606,362, 8,620,436, and U.S. Patent Application Publications2018/0071520 and 2019/0083796.

Much of the stimulation circuitry 28 of FIG. 3, including the PDACs 40_(i) and NDACs 42 _(i), the switch matrices (if present), and theelectrode nodes ei 39 can be integrated on one or more ApplicationSpecific Integrated Circuits (ASICs), as described in U.S. PatentApplication Publications 2012/0095529, 2012/0092031, and 2012/0095519.As explained in these references, ASIC(s) may also contain othercircuitry useful in the IPG 10, such as telemetry circuitry (forinterfacing off chip with telemetry antennas 27 a and/or 27 b),circuitry for generating the compliance voltage VH, various measurementcircuits, etc.

Also shown in FIG. 3 are DC-blocking capacitors Ci 38 placed in seriesin the electrode current paths between each of the electrode nodes ei 39and the electrodes Ei 16 (including the case electrode Ec 12). TheDC-blocking capacitors 38 act as a safety measure to prevent DC currentinjection into the patient, as could occur for example if there is acircuit fault in the stimulation circuitry 28. The DC-blockingcapacitors 38 are typically provided off-chip (off of the ASIC(s)), andinstead may be provided in or on a circuit board in the IPG 10 used tointegrate its various components, as explained in U.S. PatentApplication Publication 2015/0157861.

Referring again to FIG. 2A, the stimulation pulses as shown arebiphasic, with each pulse comprising a first phase 30 a followedthereafter by a second phase 30 b of opposite polarity. Biphasic pulsesare useful to actively recover any charge that might be stored oncapacitive elements in the electrode current paths, such as on theDC-blocking capacitors 38. Charge recovery is shown with reference toboth FIGS. 2A and 2B. During the first pulse phase 30 a, charge willbuild up across the DC-blockings capacitors C1 and C2 associated withthe electrodes E1 and E2 used to produce the current, giving rise tovoltages Vc1 and Vc2 which increase in accordance with the magnitude ofthe current and the capacitance of the capacitors 38 (dV/dt=I/C). Duringthe second pulse phase 30 b, when the polarity of the current I isreversed at the selected electrodes E1 and E2, the stored charge oncapacitors C1 and C2 is actively recovered, and thus voltages Vc1 andVc2 fall and hopefully return to 0V at the end the second pulse phase 30b.

To recover all charge by the end of the second pulse phase 30 b of eachpulse (Vc1=Vc2=0V), the first and second phases 30 a and 30 b arecharged balanced at each electrode, with the first pulse phase 30 aproviding a charge of +Q and the second pulse phase 30 b providing acharge of −Q. In the example shown, such charge balancing is achieved byusing the same pulse width (PW) and the same amplitude (|I|) for each ofthe opposite-polarity pulse phases 30 a and 30 b. However, the pulsephases 30 a and 30 b may also be charged balance if the product of theamplitude and pulse widths of the two phases 30 a and 30 b are equal, orif the area under each of the phases is equal, as is known.

FIG. 3 shows that stimulation circuitry 28 can include passive recoveryswitches 41 _(i), which are described further in U.S. Patent ApplicationPublications 2018/0071527 and 2018/0140831. Passive recovery switches 41_(i) may be attached to each of the electrode nodes ei 39, and are usedto passively recover any charge remaining on the DC-blocking capacitorsCi 38 after issuance of the second pulse phase 30 b—i.e., to recovercharge without actively driving a current using the DAC circuitry.Passive charge recovery can be prudent, because non-idealities in thestimulation circuitry 28 may lead to pulse phases 30 a and 30 b that arenot perfectly charge balanced. Therefore, and as shown in FIG. 2A,passive charge recovery typically occurs after the issuance of secondpulse phases 30 b, for example during at least a portion 30 c of thequiet periods between the pulses, by closing passive recovery switches41 k. As shown in FIG. 3, the other end of the switches 41 _(i) notcoupled to the electrode nodes ei 39 are connected to a common referencevoltage, which in this example comprises the voltage of the battery 14,Vbat, although another reference voltage could be used. As explained inthe above-cited references, such passive charge recovery tends toequilibrate the charge on the DC-blocking capacitors 38 by placing thecapacitors in parallel between the reference voltage (Vbat) and thepatient's tissue.

FIG. 4 shows an external trial stimulation environment that may precedeimplantation of an IPG 10 in a patient. During external trialstimulation, stimulation can be tried on a prospective implant patientwithout going so far as to implant the IPG 10. Instead, one or moretrial electrode arrays 17′ (e.g., one or more trial percutaneous leads15 or trial paddle leads 19) are implanted in the patient's tissue at atarget location 52, such as within the spinal column as explainedearlier. The proximal ends of the trial electrode array(s) 17′ exit anincision 54 in the patient's tissue and are connected to an ExternalTrial Stimulator (ETS) 50. The ETS 50 generally mimics operation of theIPG 10, and thus can provide stimulation to the patient's tissue asexplained above. See, e.g., U.S. Pat. No. 9,259,574, disclosing a designfor an ETS. The ETS 50 is generally worn externally by the patient for ashort while (e.g., two weeks), which allows the patient and hisclinician to experiment with different stimulation parameters tohopefully find a stimulation program that alleviates the patient'ssymptoms (e.g., pain). If external trial stimulation proves successful,the trial electrode array(s) 17′ are explanted, and a full IPG 10 and apermanent electrode array 17 (e.g., one or more percutaneous 15 orpaddle 19 leads) are implanted as described above; if unsuccessful, thetrial electrode array(s) 17′ are simply explanted.

Like the IPG 10, the ETS 50 can include one or more antennas to enablebi-directional communications with external devices such as those shownin FIG. 5. Such antennas can include a near-field magnetic-inductioncoil antenna 56 a, and/or a far-field RF antenna 56 b, as describedearlier. ETS 50 may also include stimulation circuitry 58 (FIG. 4) ableto form stimulation in accordance with a stimulation program, whichcircuitry may be similar to or comprise the same stimulation circuitry28 (FIG. 3) present in the IPG 10. ETS 50 may also include a battery(not shown) for operational power.

FIG. 5 shows various external devices that can wirelessly communicatedata with the IPG 10 or ETS 50, including a patient, hand-held externalcontroller 60, and a clinician programmer 70. Both of devices 60 and 70can be used to wirelessly transmit a stimulation program to the IPG 10or ETS 50—that is, to program their stimulation circuitries 28 and 58 toproduce stimulation with a desired amplitude and timing describedearlier. Both devices 60 and 70 may also be used to adjust one or morestimulation parameters of a stimulation program that the IPG 10 iscurrently executing. Devices 60 and 70 may also wirelessly receiveinformation from the IPG 10 or ETS 50, such as various statusinformation, etc.

External controller 60 can be as described in U.S. Patent ApplicationPublication 2015/0080982 for example, and may comprise a controllerdedicated to work with the IPG 10 or ETS 50. External controller 60 mayalso comprise a general purpose mobile electronics device such as amobile phone which has been programmed with a Medical Device Application(MDA) allowing it to work as a wireless controller for the IPG 10 orETS, as described in U.S. Patent Application Publication 2015/0231402.External controller 60 includes a user interface, preferably includingmeans for entering commands (e.g., buttons or selectable graphicalelements) and a display 62. The external controller 60's user interfaceenables a patient to adjust stimulation parameters, although it may havelimited functionality when compared to the more-powerful clinicianprogrammer 70, described shortly.

The external controller 60 can have one or more antennas capable ofcommunicating with the IPG 10. For example, the external controller 60can have a near-field magnetic-induction coil antenna 64 a capable ofwirelessly communicating with the coil antenna 27 a or 56 a in the IPG10 or ETS 50. The external controller 60 can also have a far-field RFantenna 64 b capable of wirelessly communicating with the RF antenna 27b or 56 b in the IPG 10 or ETS 50.

Clinician programmer 70 is described further in U.S. Patent ApplicationPublication 2015/0360038, and can comprise a computing device 72, suchas a desktop, laptop, or notebook computer, a tablet, a mobile smartphone, a Personal Data Assistant (PDA)-type mobile computing device,etc. In FIG. 5, computing device 72 is shown as a laptop computer thatincludes typical computer user interface means such as a screen 74, amouse, a keyboard, speakers, a stylus, a printer, etc., not all of whichare shown for convenience. Also shown in FIG. 5 are accessory devicesfor the clinician programmer 70 that are usually specific to itsoperation as a stimulation controller, such as a communication “wand” 76coupleable to suitable ports on the computing device 72, such as USBports 79 for example.

The antenna used in the clinician programmer 70 to communicate with theIPG 10 or ETS 50 can depend on the type of antennas included in thosedevices. If the patient's IPG 10 or ETS 50 includes a coil antenna 27 aor 56 a, wand 76 can likewise include a coil antenna 80 a to establishnear-filed magnetic-induction communications at small distances. In thisinstance, the wand 76 may be affixed in close proximity to the patient,such as by placing the wand 76 in a belt or holster wearable by thepatient and proximate to the patient's IPG 10 or ETS 50. If the IPG 10or ETS 50 includes an RF antenna 27 b or 56 b, the wand 76, thecomputing device 72, or both, can likewise include an RF antenna 80 b toestablish communication at larger distances. The clinician programmer 70can also communicate with other devices and networks, such as theInternet, either wirelessly or via a wired link provided at an Ethernetor network port.

To program stimulation programs or parameters for the IPG 10 or ETS 50,the clinician interfaces with a clinician programmer graphical userinterface (GUI) 82 provided on the display 74 of the computing device72. As one skilled in the art understands, the GUI 82 can be rendered byexecution of clinician programmer software 84 stored in the computingdevice 72, which software may be stored in the device's non-volatilememory 86. Execution of the clinician programmer software 84 in thecomputing device 72 can be facilitated by control circuitry 88 such asone or more microprocessors, microcomputers, FPGAs, DSPs, other digitallogic structures, etc., which are capable of executing programs in acomputing device, and which may comprise their own memories. Forexample, control circuitry 88 can comprise an i5 processor manufacturedby Intel Corp, as described athttps://www.intel.com/content/www/us/en/products/processors/core/i5-processors.html.Such control circuitry 88, in addition to executing the clinicianprogrammer software 84 and rendering the GUI 82, can also enablecommunications via antennas 80 a or 80 b to communicate stimulationparameters chosen through the GUI 82 to the patient's IPG 10.

The user interface of the external controller 60 may provide similarfunctionality because the external controller 60 can include the samehardware and software programming as the clinician programmer. Forexample, the external controller 60 includes control circuitry 66similar to the control circuitry 88 in the clinician programmer 70, andmay similarly be programmed with external controller software stored indevice memory.

SUMMARY

In a first example, a stimulator device is disclosed which may comprise:a plurality of electrode nodes, each electrode node configured to becoupled to one of a plurality of electrodes configured to contact apatient's tissue; and stimulation circuitry configured to provide in asingle timing channel a sequence of pulses at at least two of theelectrode nodes selected to create a stimulation current through thepatient's tissue, wherein the stimulation circuitry is configured toform each pulse at the selected electrode nodes with a first phase and asecond phase, wherein the first phase at each selected electrode nodecomprises a plurality of first monophasic sub-phase pulses of a firstpolarity and separated by gaps during which no current is issued to thetissue, wherein the second phase at each selected electrode nodecomprises a plurality of second monophasic sub-phase pulses of a secondpolarity opposite the first polarity and separated by gaps during whichno current is issued to the tissue, and wherein at each selectedelectrode node a first total charge of the plurality of first monophasicsub-phase pulses is equal but opposite a second total charge of theplurality of second monophasic sub-phase pulses.

The stimulator device may further comprise a case for housing thestimulation circuitry, wherein the case is conductive, and wherein theconductive case comprises one of the plurality of electrodes. At leastone selected electrode node may be coupled to its associated electrodethrough a DC-blocking capacitor. The stimulator device may furthercomprise at least one implantable lead, wherein the electrodes arelocated on the lead.

Each first and second monophasic sub-phase pulse may be of a constantamplitude, which may comprises a constant current. An amplitude of thefirst monophasic sub-phase pulses may vary during the first pulse phase,or an amplitude of the second monophasic sub-phase pulses may varyduring the second pulse phase. The amplitude of the first monophasicsub-phase pulses may vary during the first pulse phase and the amplitudeof the second monophasic sub-phase pulses may vary during the secondpulse phase. A pulse width of the first monophasic sub-phase pulses mayvary during the first pulse phase, or a pulse width of the secondmonophasic sub-phase pulses may vary during the second pulse phase. Thepulse width of the first monophasic sub-phase pulses may varies duringthe first pulse phase and the pulse width of the second monophasicsub-phase pulses may vary during the second pulse phase. A frequency ofthe first monophasic sub-phase pulses may vary during the first pulsephase, or a frequency of the second monophasic sub-phase pulses may varyduring the second pulse phase. The frequency of the first monophasicsub-phase pulses may vary during the first pulse phase and the frequencyof the second monophasic sub-phase pulses may vary during the secondpulse phase.

The stimulator device may further comprise control circuitry, whereinthe control circuitry is configured to receive a plurality ofstimulation parameters including a first frequency of the pulses, asecond frequency of the first and second monophasic sub-phase pulses, apulse width of at least one of the first and second phases, and a pulsewidth of the first and second monophasic pulses, and the controlcircuitry may be configured to use the stimulation parameters to providea plurality of control signals to the stimulation circuitry to cause thestimulation circuitry to form the sequence of pulses at the selectedelectrode nodes. The stimulator device may further comprise an antenna,wherein the control circuitry is configured to receive the stimulationparameters from the antenna. The control circuitry may be configured toproduce a first digital signal at the second frequency, a second digitalsignal at the first frequency and corresponding to a timing of the firstphase, and a third digital signal at the first frequency andcorresponding to a timing of the second phase. The stimulation circuitrymay comprise a plurality of switches, wherein the plurality of switchesare controlled by a mixture of the first and second digital signalsduring the first phase, and wherein the plurality of switches arecontrolled by a mixture of the first and third digital signals duringthe second phase.

The stimulator device may comprise an implantable pulse generator or anexternal stimulator.

The stimulation circuitry may be configured to form an interphase periodat the selected electrode nodes between the first phase and the secondphase, wherein no current is issued to the tissue during the interphaseperiod.

The first monophasic sub-phase pulses may be positive at at least one ofthe selected electrode nodes and negative at at least one other of theselected electrode nodes such that the net current injected into thetissue at any time is zero during the first phase, and the secondmonophasic sub-phase pulses may be negative at the at least one of theselected electrode nodes and positive at the at least one other of theselected electrode nodes such that the net current injected into thetissue at any time is zero during the second phase.

In a second example, a stimulator device is disclosed which maycomprise: a plurality of electrode nodes, each electrode node configuredto be coupled to one of a plurality of electrodes configured to contacta patient's tissue; and stimulation circuitry configured to provide asequence of pulses at at least two of the electrode nodes selected tocreate a stimulation current through the patient's tissue, wherein thestimulation circuitry is configured to form each pulse at the selectedelectrode nodes with a first phase and a second phase, wherein one ofthe first or second phases at each selected electrode node comprises aplurality of monophasic sub-phase pulses of a first polarity andseparated by first gaps, wherein a non-zero current of the firstpolarity is provided during the first gaps, wherein at each selectedelectrode node a first total charge of the plurality of monophasicsub-phase pulses plus the non-zero current is equal but opposite asecond total charge of the second phase.

The stimulator device may further comprising a case for housing thestimulation circuitry, wherein the case is conductive, and wherein theconductive case comprises one of the plurality of electrodes. At leastone selected electrode node may be coupled to its associated electrodethrough a DC-blocking capacitor. The stimulator device may furthercomprise at least one implantable lead, wherein the electrodes arelocated on the lead.

Each monophasic sub-phase pulses may be of a constant amplitude, whichmay comprise a constant current. The other of the first or second phasesat each selected electrode node may comprise a plurality of monophasicsub-phase pulses of a second polarity opposite the first polarity andseparated by second gaps, wherein a non-zero current of the secondpolarity is provided during the second gaps.

The other of the first or second phases at each selected electrode nodemay comprise a constant pulse.

An amplitude of the monophasic sub-phase pulses may vary during the oneof the first or second phases. The non-zero current provided during thefirst gaps may be constant during the one of the first or second phases.The non-zero current provided during the first gaps may also vary duringthe one of the first or second phases. A pulse width of the monophasicsub-phase pulses may vary during the one of the first or second phases.A frequency of the monophasic sub-phase pulses may vary during the oneof the first or second phases.

The stimulator device may further comprise control circuitry, whereinthe control circuitry is configured to receive stimulation parametersincluding a first frequency of the pulses, a second frequency of themonophasic sub-phase pulses, a pulse width of at least one of the firstand second phases, and a pulse width of the monophasic pulses, whereinthe control circuitry is configured to use the stimulation parameters toprovide a plurality of control signals to the stimulation circuitry tocause the stimulation circuitry to form the sequence of pulses at theselected electrode nodes. The stimulator device may further comprise anantenna, wherein the control circuitry is configured to receive thestimulation parameters from the antenna.

The stimulator device may comprises an implantable pulse generator or anexternal stimulator.

The stimulation circuitry may be configured to form an interphase periodat the selected electrode nodes between the first phase and the secondphase, wherein no current is issued to the tissue during the interphaseperiod.

The monophasic sub-phase pulses may be positive at at least one of theselected electrode nodes and negative at at least one other of theselected electrode nodes such that the net current injected into thetissue at any time is zero during the one of the first or second phases.

The stimulation circuitry may be configured to provide the sequence ofpulses in a single timing channel.

In a third example, a stimulator device is disclosed, which maycomprise: a plurality of electrode nodes, each electrode node configuredto be coupled to one of a plurality of electrodes configured to contacta patient's tissue; and stimulation circuitry configured to provide in asingle timing channel a sequence of pulses at at least two of theelectrode nodes selected to create a stimulation current through thepatient's tissue, wherein the stimulation circuitry is configured toform each pulse at the selected electrode nodes with a first phase and asecond phase, wherein the first phase at each selected electrode nodecomprises a plurality of first monophasic sub-phase pulses of a firstpolarity and separated by first gaps, wherein the second phase at eachselected electrode node comprises a plurality of second monophasicsub-phase pulses of a second polarity opposite the first polarity andseparated by second gaps, and wherein at each selected electrode node afirst total charge of the first phase is equal but opposite a secondtotal charge of the second phase.

No current may be issued to the current during the first and secondgaps. A non-zero current of the first polarity may be provided duringthe first gaps, and a non-zero current of the second polarity may beprovided during the second gaps. Alternatively, a non-zero current ofthe same polarity may be provided during the first and second gaps, andthe non-zero current may have the same amplitude during the first andsecond gaps.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an Implantable Pulse Generator (IPG), in accordance withthe prior art.

FIGS. 2A and 2B show an example of stimulation pulses (waveforms)producible by the IPG or by an External Trial Stimulator (ETS), inaccordance with the prior art.

FIG. 3 shows an example of stimulation circuitry useable in the IPG orETS, in accordance with the prior art.

FIG. 4 shows an ETS environment useable to provide stimulation beforeimplantation of an IPG, in accordance with the prior art.

FIG. 5 shows various external devices capable of communicating with andprogramming stimulation in an IPG or ETS, in accordance with the priorart.

FIGS. 6A-6C show biphasic waveforms producible by the IPG or ETS at lowand high frequencies, and in pulse packets.

FIG. 7 shows a first example of waveforms producible by the IPG or ETSat electrodes and having both low- and high-frequency aspects inaccordance with the invention, having charge-balanced first and secondpulse phases, in which each pulse phase is comprised of high-frequencymonophasic sub-pulses.

FIG. 8 shows a Graphical User Interface (GUI) of an external device incommunication with the IPG or ETS, including settings to allow forprogramming stimulation parameters for the waveforms of FIG. 7.

FIG. 9 shows circuitry in the IPG or ETS capable of receiving thestimulation parameters from the GUI and for forming the waveforms ofFIG. 7.

FIG. 10 shows alternative circuitry in the IPG for forming the waveformsof FIG. 7, which circuitry mixes low- and high-frequency digital signalsand uses such mixed signals to control the stimulation circuitry in theIPG.

FIGS. 11A-11C show variations to the waveforms of FIG. 7.

FIGS. 12A and 12B show second examples of waveforms producible by theIPG or ETS at electrodes and having both low- and high-frequency aspectsin accordance with the invention, having charge-balanced first andsecond pulse phases, in which each pulse phase is comprised ofhigh-frequency monophasic sub-pulses which do not return to zero duringthe gaps between the sub-pulses.

FIGS. 13A-13F show variations to the waveforms of FIGS. 12A and 12B.

FIGS. 14A and 14B show examples of waveforms in which active firstpulses phases are followed by passive recovery phases, and FIG. 14Cshows passive recovery circuitry.

DETAILED DESCRIPTION

Traditionally, pain relief in Spinal Cord Stimulation (SCS) systems wasachieved by using a sequence of pulses operating at a relatively lowfrequency, f_(L), as shown in FIG. 6A. FIG. 6A shows a stimulationprogram essentially similar to that illustrated in FIG. 2A, in whichbiphasic pulses with phases 100 a and 100 b are used to create a currentI between any electrodes (e.g., E1 and E2) selected from the pluralityof electrodes. Here it is assumed that the duration PWa of both phases100 a and 100 b of the pulses are the same as well as their amplitudes Ifor balanced active charge recovery, although these pulsewidths/amplitudes can differ while still achieving charge-balancedphases, as explained earlier. Also shown is an interphase period (IP)between the phases 100 a and 100 b in each pulse, during which nocurrent is provided; this is useful to provide time to allow switchingin the stimulation circuitry 28/58 in the IPG or ETS to stabilize afterexiting a first pulse phase 100 a and before entering the second pulsephase 100 b. Further shown are the passive charge recovery periods 100 cthat can occur during quiet periods 100 d when passive recovery switches41 _(i) (FIG. 3) can be closed, again as explained earlier. Generallyspeaking, the low frequency f_(L) of the pulses in FIG. 6A can comprise200 Hz or less.

The use of lower-frequency pulses for spinal cord stimulation has beenreported by patients, in addition to pain relief, to sometimes causeparesthesia, i.e., a tickling, prickling, or temperature-changesensation. While some patients don't mind, or may actually enjoy, thefeeling of paresthesia, other patients would prefer to not feelparesthesia. High-frequency stimulation has been reported as helpful inreducing paresthesia perhaps to sub-threshold levels, as shown in FIG.6B. Here, biphasic pulses with charge-balanced phases 102 a and 102 bare issued at a higher frequency f_(H), which can comprise 2000 Hz orgreater. Because the pulses are issued at a high frequency, their pulsewidths are generally smaller as well, and in FIG. 6B a single pulsewidth PWb is shown for both of the phases 102 a and 102 b, althoughagain this is not strictly necessary as charge balance between thephases can be achieved in different manners.

While high-frequency pulses such as those shown in FIG. 6B may behelpful in reducing or eliminating paresthesia, such pulses also presentimplementation challenges in an IPG 10 or ETS 50. First, the quietperiod 102 d between the pulses—i.e., after a second pulse phase 102 band before a next first pulse phase 102 a—is short. This can make use ofpassive charge recovery during periods 102 c difficult or ineffective,either because there is not enough time to close the passive recoveryswitches 41 _(i) (FIG. 3), or because the time period during which thoseswitches can be closed is not long enough to allow the DC-blockingcapacitors 38 (FIG. 3) to equilibrate charge that may be stored on them.See, e.g., U.S. Pat. No. 10,792,491, discussing this problem in furtherdetail.

Second, it can also be difficult to accommodate a significant interphaseperiod (IP) between each of pulses phases 102 a and 102 b, because againthere may not be sufficient time to implement such a period. This canmake switching in the stimulation circuitry 28/58 difficult, and canlead to unwanted ringing in the pulse phases.

High-frequency pulses as shown in FIG. 6B also require more power toproduce. As one skilled in the art will appreciate, higher-frequencypulses require more frequent switching in the stimulation circuitry28/58 used to form the pulses. This leads to higher power consumptioncompared to lower-frequency pulses that require less frequent switching,even if the time-average of the current is the same for the lower andhigher frequency pulses (e.g., if they have the same on/off duty cycle).This is especially problematic for an IPG 10 powered by a battery 14,which is implanted and cannot be changed. High-frequency pulses willcause a permanent battery 14 to deplete more quickly, which couldrequire explantation of the IPG 10 before its useful life is otherwisespent. If the battery 14 is rechargeable, high-frequency pulses willrequire more frequent external (wireless) charging of the battery,possibly to a point where such charging becomes inconvenient or isimpractical. The need to more frequently externally charge arechargeable battery 14 also accelerates the battery's degradation,again running the risk of the need for a premature explantation.

In FIG. 6B, the high-frequency biphasic pulses are free running,effectively issuing without cessation over some therapeutic time period.One manner of mitigating increased power consumption when usinghigh-frequency pulses is to issue such pulses in packets 103, as shownin FIG. 6C. As shown, a number of biphasic pulses pulse of highfrequency f_(H) are issued in each packet 103, with each packet 103being followed by a period 104 of no current (stimulation). This reducespower consumption, both because the time-averaged current is reduced (nocurrent issues during periods 104), and because the number of times thestimulation circuitry 28/58 must be switched is reduced. However, it canstill be difficult to provide passive charge recovery 102 c during thequiet 102 d periods between the biphasic pulses in each packet 103, andagain it is difficult to provide a significant interphase period (IP) toassist with switching between pulse phases 102 a and 102 b.

To remedy these problems while still providing reduced paresthesia(sub-threshold) stimulation to a patient, new stimulation waveforms, andmethods and circuitry for producing them, are disclosed having bothlow-frequency (f_(L)) and high-frequency (f_(H)) features. A firstexample of such a waveform is shown in FIG. 7. FIG. 7 shows a sequenceof pulses, with each pulse comprising a first phase 106 a followed by asecond phase 106 b. These pulses are issued at a low frequency f_(L),similar to the pulses of FIG. 6A. The duration PW_(L) of each of pulsephases 106 a and 106 b may equal the duration PW_(H) of the pulse phases100 a and 100 b of the low-frequency biphasic pulses of FIG. 6A, butthis is not strictly necessary.

Each pulse phase 106 a and 106 b is fractionalized into sub-phase pulses107 a and 107 b. Each of the sub-phase pulses 107 a and 107 b are issuedat a high frequency f_(H), similar to the high frequency at which thebiphasic pulses are issued in FIGS. 6B and 6C. Notice however that thesub-phase pulses 107 a and 107 b are monophasic, not biphasic. Thus, atelectrode E1, each sub-phase pulse 107 a is only positive during firstpulse phases 106 a, and each sub-phase pulse 107 b is only negativeduring second pulse phases 106 b. At electrode E2 the polarity isreversed, such that each sub-phase pulse 107 a is only negative duringfirst pulse phases 106 a, and each sub-phase pulse 107 b is onlypositive during second pulse phases 106 b.

In the illustrated example, each of the sub-phase pulses 107 a and 107 bhas the same amplitude +I or −I and the same pulse width PW_(H).L Notethat at any time the current sourced to the tissue (e.g., +I at E1during sub-phase pulses 107 a) equals the current sunk from the tissue(e.g., −I at E2 during sub-phase pulses 107 a) to ensure that the netcurrent injected into the tissue at any time is zero.

The pulse width PW_(H) of the sub-phase pulses 107 a and 107 b may equalthe duration of either of the pulse phases 102 a or 102 b (e.g., PWb),or of the total duration of both phases (e.g., 2 PWb), of thehigh-frequency biphasic pulses shown in FIGS. 6B and 6C. However, thisis not strictly necessary.

The sub-phase pulses 107 a and 107 b are separated by gaps 108 ofduration t_(H), during which, in one example, no stimulation currentoccurs. The duration t_(H) of gaps 108 may equal the duration of thequiet periods 102 d between the high-frequency biphasic pulses shown inFIGS. 6B and 6C, but again this is not strictly necessary. The durationof the sub-phase pulses 107 a and 107 b (PW_(H)) may equal the durationof the gaps 108 (t_(H)) as shown, but again this is not necessary.

In a preferred example, the total charge provided by first pulse phase106 a at each electrode equals the opposite total charge provided by thesecond pulse phase 106 b. In other words, the total charge +Q providedby the sub-phase pulses 107 a of the first pulse phase 106 a atelectrode E1 equals the opposite total charge −Q provided by thesub-phase pulses 107 b of the second pulse phase 106 b at electrode E1,and likewise for electrode E2 but with the charge polarity reversed.This provides charge balance to the pulses at each electrode.

Notice that while the sub-phase pulses 107 a and 107 b are issued at ahigh frequency f_(H), the fact that they are not biphasic means that thenumber of times the stimulation circuitry 28/58 must be switched isreduced. For example, and by comparison to the high frequency biphasicpulses of FIGS. 6B and 6C, the frequency of switching can be reduced byhalf: whereas each high-frequency biphasic pulse at an electrode mustswitch on, then off (assuming an interphase period is used), then to theopposite polarity, then off, each sub-phase pulse 107 a or 107 b willonly switch on and then off during the same duration. This saves power,and is thus more considerate of the IPG or ETS battery.

As shown in FIG. 7, an interphase period (IP) may intervene between thefirst and second pulse phases 106 a and 106 b, and passive recovery may(closing of switches 41 _(i); FIG. 3) occur during periods 106 cfollowing the second pulse phases 106 b, which periods 106 c can occurfor at least a portion of quiet periods 106 d between the pulses. Theduration of the passive recovery periods 106 c may equal the duration ofsimilar periods 100 c in the low-frequency pulses of FIG. 6A, althoughthis isn't necessary. Likewise, the quiet periods 106 d between thepulses may equal the duration of the similar periods 106 d in thelow-frequency pulses of FIG. 6A, or may equal the duration 104 betweenthe packets 103 of high-frequency pulses in FIG. 6C, although again thisisn't necessary. Unlike the high-frequency biphasic pulses used in FIGS.6B and 6C, the low-frequency, longer-time-scale aspects of the waveformsof FIG. 7 provide ample time to accommodate the interphase period (IP)and passive recovery during periods 106 c.

While the pulses of FIG. 7 desirably provide aspects of high-frequencyparesthesia-free stimulation, they behave like low frequency pulses froma charge injection standpoint. This is illustrated by reviewing themanner in which charge accumulates on the DC-blocking capacitors 38(FIG. 3) during each of the pulses (Vc1, Vc2). For the low-frequencypulses of FIG. 6A, and as already explained (FIG. 2A), chargeaccumulates on the capacitors during the first pulse phases 100 a (Vc1and Vc2 increase) and is actively recovered during the second pulsephases 100 b (as Vc1 and Vc2 decrease back to zero). For thehigh-frequency pulses of FIGS. 6B and 6C, essentially the same processoccurs, but on a faster time scale due to the higher-frequency bi-phasicpulses.

For the pulses of FIG. 7, charge is injected during each sub-phase pulse107 a during the first pulse phases 106 a, causing Vc1 and Vc2 toincrease. Vc1 and Vc2 remain constant (or may slightly decay) during thegaps 108 when no current is provided. Charge is then actively recoveredduring each sub-phase pulse 107 b during each second pulse phase 106 b,causing Vc1 and Vc2 to decrease (except during the gaps 108) and returnto zero at the end of the second pulse phase 106 b. Notice that theoverall charge injection profile in FIG. 7 as evidenced by Vc1 and Vc2is similar in shape to, and occurs on the same time scale as, thelow-frequency pulses of FIG. 6A. This is preferable to thehigher-frequency charge injection that occurs in the high-frequencypulses of FIGS. 6B and 6C. As explained in the '491 Patent referencedabove, the short time scales of high-frequency biphasic pulses can makecharge recovery difficult.

Preferably the waveforms of FIG. 7 are formed in a single timingchannel, i.e., with each of the sub-phase pulses 107 a and 107 b at anelectrode being defined and formed in a single timing channel. This isdifferent, and more convenient, than forming some of the sub-phasepulses 107 a and 107 b in different timing channels and combining themat the electrode, as described in U.S. Pat. No. 9,358,391 for example.Plus, forming the sub-phase pulses in a single timing channel frees theother timing channels in the IPG or ETS, which may now be used toprovide pulses at different electrodes, therefore allowing more complextherapies to be provided to the patient. Use of timing channels in anIPG is discussed further in U.S. Pat. Nos. 6,516,227 and 9,656,081,which are incorporated herein by reference.

FIG. 8 shows a Graphical User Interface (GUI) 120 which can be used toprogram an IPG 115 to provide the pulses of FIG. 7. GUI 120 may beprovided on an external device, such as the external controller 60 orclinician programmer 70 of FIG. 5. One skilled in the art willunderstand that the particulars of the GUI 120 will depend on where theexternal device's software is in its execution, which may depend on theGUI selections the clinician or patient has previously made. Theinstructions for GUI 120 can be stored on a non-transitory computerreadable media, such as a solid state, optical, or magnetic memory, andloaded into the relevant external device.

FIG. 8 shows the GUI 120 at a point allowing for the manual setting ofstimulation parameters for the patient. A stimulation parametersinterface 122 is provided in which specific stimulation parameters canbe defined for a stimulation program. Adjustable settings forstimulation parameters are shown, including the amplitude I of thestimulation pulses, and, as particularly relevant to the pulses of FIG.7, settings to adjust the low (f_(L)) and high (f_(H)) frequency aspectsof the pulses. Pulse widths PW_(L) and PW_(H) are also provided to setthe duration of the pulse phases 106 a/ 106 b and the duration of eachof the sub-pulse phases 107 a/ 107 b respectively. GUI 120 assumes forsimplicity that PW_(L) will be the same for each of the pulse phases 106a and 106 b and that and PW_(H) will be the same for the sub-phasepulses 107 a and 107 b, but this isn't necessary, and instead means canbe provided to allow these parameters to be set separately. The durationt_(H) of the gaps 108 could also be made adjustable in the stimulationparameters interface 122, but this isn't shown for simplicity.

Stimulation parameters relating to the electrodes 16 are made adjustablein an electrode parameter interface 124. Electrodes are manuallyselectable in a leads interface 126 that displays a graphicalrepresentation of the electrode array 17 or 17′ (one or more permanentor trial leads) that has been implanted in a particular patient (apaddle lead 19 is shown as one example). A cursor 128 (or otherselection means such as a mouse pointer) can be used to select aparticular electrode in the leads interface 126. Buttons in theelectrode parameter interface 124 allow the selected electrode(including the case electrode, Ec) to be designated as an anode, acathode, or off. The electrode parameter interface 124 further allowsthe amount of the total anodic or cathodic current +I or −I that eachselected electrode will receive to be specified in terms of apercentage, X. For example, for the waveforms of FIG. 7, the electrodeparameter interface 124 can specify that electrode E1 will receiveX=100% of the current I as an anodic current +I (during the first pulsephase 106 a) and that electrode E2 will receive X=100% of the current Ias a cathodic current −I (again during the first pulse phase 106 a).Note that two or more electrodes can be chosen to act as anodes orcathodes at a given time. For example and although not illustrated,suppose E1 will act as an anode, and that electrodes E2 and E3 will actas cathodes at a given time. In this circumstance, the electrodeparameter interface 124 can specify that electrode E1 will receiveX=100% of the current I as an anodic current +I, that electrode E2 (forexample) will receive X=70% of the current I as a cathodic current−0.71, and that electrode E3 will receive the remaining X=30% of thecurrent I as a cathodic current −0.31.

FIG. 9 shows an illustration of an IPG or ETS 115 capable of forming thepulses of FIG. 7. As discussed in the Introduction, the stimulationparameters entered from the GUI 120 of FIG. 8 can be wirelesslytransmitted by the external device 60 or 70 to an antenna in the IPG orETS 115, including the amplitude I, low frequency f_(L), high frequencyf_(H), low-frequency pulse width PW_(L), high-frequency pulse widthPW_(H), the anode and cathode electrodes (A and C) selected to receivethe stimulation pulses, and the relative percentage X of amplitude eachanode and cathode is to receive (not shown in FIG. 9 for simplicity).

In FIG. 9, the stimulation parameters, once wirelessly received, areprovided to control circuitry 140. Control circuitry 140 may comprise amicrocontroller for example, such as Part Number MSP430, manufactured byTexas Instruments, which is described in data sheets athttp://www.ti.com/lsds/ti/microcontroller/16-bit msp430/overview.page?DCMP=MCU_other& HQS=msp430. The control circuitry 140 more generally cancomprise a microprocessor, Field Programmable Grid Array, ProgrammableLogic Device, Digital Signal Processor or like devices. Controlcircuitry 140 may also be based on well-known ARM microcontrollertechnology. Control circuitry 140 may include a central processing unitcapable of executing instructions, with such instructions stored involatile or non-volatile memory within or associated with the controlcircuitry. Control circuitry 140 may also include, operate inconjunction with, or be embedded within an Application SpecificIntegrated Circuit (ASIC), such as described in U.S. Patent ApplicationPublications 2008/0319497, 2012/0095529, 2018/0071513, or 2018/0071520,which are incorporated herein by reference. The control circuitry 140may comprise an integrated circuit with a monocrystalline substrate, ormay comprise any number of such integrated circuits operating as asystem. Control circuitry may also be included as part of aSystem-on-Chip (SoC) or a System-on-Module (SoM) which may incorporatememory devices and other digital interfaces.

In FIG. 9, the control circuitry 140 includes pulse logic 142, whichreceives the stimulation parameters and forms various control signals144 for the stimulation circuitry 28/58. Such control signals 144specify the timing and polarity of the stimulation pulses appearing ateach of the selected electrodes, as well as the amplitude of the currenteach selected electrode will provide. As relevant to forming thewaveforms of FIG. 7, the pulse logic 142 will in particular receive theinformation relevant to the timing of the waveforms, e.g., f_(L), f_(H),PW_(L), and PW_(H), and use this information to form the waveforms withthe prescribed timing.

FIG. 10 shows another example of circuitry that can be used to form thepulses of FIG. 7 in IPG or ETS 115. In this example, the controlcircuitry 140 outputs a high-frequency digital signal H andlow-frequency digital signals La and Lb with the appropriate timings andmixes them. The mixed signals are then used to control switches 146 _(i)and 148 _(i) added to the stimulation circuitry 28/58.

Before describing the digital signals, the stimulation circuitry 28/58as modified is described. As just mentioned, the stimulation circuitry28/58 includes a switch 146 _(i) in the current path between a givenPDAC 40 _(i) and a given DC-blocking capacitor Ci, and a switch 148 _(i)in the current path between a given NDAC 42 _(i) and the DC-blockingcapacitor Ci. This establishes different electrode nodes 39 _(p) foreach PDAC output (ei_(p)) and different electrodes nodes 39 p for eachNDAC output (ei_(p)). Passive recovery switches 41 _(i) are connectedbetween the switches 146 _(i) and 148 _(i) and as before are coupled tothe inside plate of the DC-blocking capacitors 38.

Control circuitry 140 forms a high-frequency digital signal H, which isshown for simplicity as a free running signal. However, this is notstrictly necessary, as H may instead only be issued at times that thestimulation circuitry 28/58 is scheduled to issue pulses—i.e., duringlow-frequency signals La and Lb, as explained further below.High-frequency signal H is formed at high frequency f_(H) with pulsewidth PW_(H), leaving gaps of duration t_(H) as explained previously.

Control circuitry 140 also forms low-frequency digital signals La andLb, at a low frequency f_(L) with pulse width PW_(L). The timing of Laand Lb correspond to the timing of first pulse phase 106 a and secondpulse phase 106 b respectively.

Logic circuitry 149 receives H, La and Lb, and forms control signals forthe switches 146, coupled to the PDACs and switches 148, coupled to theNDACs. (Logic circuitry 149 may be implemented and comprise part ofcontrol circuitry 140). Specifically, switch 146 ₁ is controlled bycontrol signal 1P; switch 146 ₂ is controlled by control signal 2P; etc.Switch 148 ₁ is controlled by control signal 1N; switch 148 ₂ iscontrolled by control signal 2N; etc. Digital signals H and La are mixedto form a digital signal Ma having both the high- and low-frequencytiming information, which mixing can be achieved using an AND gate 150a. Likewise, digital signals H and Lb are mixed to form digital signalMb using AND gate 150 b.

Mixed signal Ma is used to control switches 146, and/or 148, during thefirst pulse phase 106 a, while mixed signal Mb is used to control theseswitches during the second pulse phase 106 b. To send Ma and Mb to thecorrect switches, multiplexers (MUXes) 152 a and 152 b are used. BothMUXes 152 a and 152 b are controlled in accordance with the electrodesselected to act as anodes or cathodes during the pulses phases 106 a and106 b (A,C).

Thus, to form the pulses of FIG. 7, MUX 152 a is informed that electrodeE1 will act as an anode during the first pulse phase 106 a, and thatelectrode E2 will act as a cathode during the first pulse phase 106 a.This will cause MUX 152 a to pass Ma to outputs control signals 1P and2N, which will open and close switches 146 ₁ and 148 ₂ during the firstpulse phase 106 a and creating the monophasic sub-phase pulses 107 aduring this period. Note that control circuitry 140 has programmed PDAC40 ₁ associated with switch 146 ₁ and NDAC 42 ₂ associated with switch148 ₂ with the prescribed amplitude I so that the sub-phase pulses 107 awill form at electrodes E1 and E2 with the correct amplitude.

Similarly, MUX 152 b is informed that electrode E1 will act as a cathodeduring the second pulse phase 106 b, and that electrode E2 will act asan anode during the second pulse phase 106 a. This will cause MUX 152 bto pass Mb to outputs control signals 1N and 2P, which will open andclose switches 146 ₂ and 148 ₁ during the second pulse phase 106 b, thuscreating the monophasic sub-phase pulses 107 b during this period.Again, control circuitry 140 has programmed PDAC 40 ₂ associated withswitch 146 ₂ and NDAC 42 ₁ associated with switch 148 ₁ with theprescribed amplitude I.

FIGS. 11A-11C show other examples of stimulation waveforms having bothlow frequency (f_(L)) and high frequency (f_(H)) features. Forsimplicity, the waveform at only a single selected electrode (e.g., E1)is shown, although it should be understood that one or more electrodes(e.g., E2) would also be active and of the opposite polarity to ensurethat the net current injected into the tissue at any time is zero, asoccurred in FIG. 7.

The waveforms of FIG. 11A and 11B are similar to those of FIG. 7 in thateach pulse is issued at a low frequency f_(L) and comprises a firstphase 160 a followed by a second phase 160 b. Each pulse phase 160 a and160 b is also fractionalized into monophasic sub-phase pulses 161 a and161 b issued at a high frequency f_(H), which sub-phase pulses areseparated by gaps 162. Although not shown, the same durations describedearlier with reference to FIG. 7 (PW_(L), PW_(H), t_(H), etc.) can applyto the phases and sub-phases shown in FIG. 11.

In FIG. 11A, the current amplitudes of the sub-phase pulses 161 a and161 b are not constant over the duration of the first and second pulsephases 160 a and 160 b, but instead vary. The variation in theamplitudes is similar during phases 160 a and 160 b (with each rampingup and the down), but this isn't required. For example, the amplitudesof sub-phase pulses 161 a during the first pulse phase 160 a could vary,while the amplitudes of sub-phase pulses 161 b during the second pulsephase 160 b could vary differently or be constant. Nonetheless, the twophases 160 a and 160 b are charge balanced at each electrode, i.e., withthe total charge provided by the sub-phase pulses 161 a equaling +Qduring the first pulse phase 160 a and the total charge provided by thesub-phase pulses 161 b equaling −Q during the second pulse phase 160 b.

In FIG. 11B, the current amplitudes of the sub-pulses 161 a and 161 bare constant over the duration of the first and second pulse phases 160a and 160 b, but the high-frequency pulse widths PW_(H) of the sub-phasepulses 161 a and 161 b vary during each of the pulse phases. Thevariation in the pulse width PW_(H) is similar during phases 160 a and160 b, but again this isn't required. For example, the pulse widthPW_(H) of sub-phase pulses 161 a during the first pulse phase 160 acould vary, while the pulse width PW_(H) of sub-phase pulses 161 bduring the second pulse phase 160 b could vary differently or beconstant. Nonetheless, the two phases 160 a and 160 b are chargebalanced at each electrode, i.e., with the charge provided by thesub-phase pulses 161 a equaling +Q during the first pulse phase 160 aand the charge provided by the sub-phase pulses 161 b equaling −Q duringthe second pulse phase 160 b. Note also that the high frequency of thesub-phase pulses 161 a and 161 b may vary within each of the pulsephases 160 a and 160 b.

In FIG. 11C, charge balancing at each electrode is provided even thoughsub-phase pulses are provided during only one of the pulse phases 160 aor 160 b. For example, in FIG. 11C, only first pulse phase 160 a hasmonophasic sub-phase pulses 161 a (of total charge +Q); the second pulsephase 160 b comprises a constant pulse 110 a or 110 b (of charge −Q).The duration or amplitude of the constant pulse 110 a or 110 b can vary.For example, constant pulse 110 a has a pulse width equal to theduration of the first pulse phase 160 a, but an amplitude −I′ that issmaller than the amplitude +I of the sub-phase pulses 161 a. Constantpulse 110 b has an amplitude −I equal to the amplitude +I of thesub-phase pulses 161 a, but has a pulse width that is smaller than theduration of the first pulse phase 160 a. In either case, the charged isbalanced during both pulses phases 160 a and 160 b (+Q=|−Q|).

FIG. 12A shows another example of stimulation waveforms having both lowfrequency (f_(L)) and high frequency (f_(H)) features. Different in thewaveforms of FIG. 12A are current levels provided during the gaps 172between the sub-phase pulses 171 a and 171 b. In FIG. 7, the currentreturned to zero during the gaps 108 between the sub-phase pulses 107 aand 107 b. In FIG. 12A however, the current does not return to zeroduring the gaps 172, but instead returns to a smaller magnitude currentof the same polarity as the sub-phase pulses 171 a and 171 b. Thus,during first phase 170 a, electrode E1's sub-phase pulses 171 a arepositive, and have a magnitude of +I1. During the gaps 172, the currentreturns to a smaller positive value of +I2. Electrode E2's waveformduring the first phase 170 a is similar, but of opposite polarity toensure that a net amount of current is not delivered to the patient'stissue. E2's sub-phase pulses 171 a are thus negative with a magnitudeof −I1, with the current returning to a smaller negative value of −I2.During the second pulse phase 170 b, electrode E1's sub-phase pulses 171b are negative with a magnitude of −I1, and returning to −I2 during thegaps 172; electrode E2's sub-phase pulses 171 b are positive with amagnitude of +I1, and returning to +I2 during the gaps 172. A passiverecovery period 170 c can occur during quiet periods between the pulsesas before. Preferably the waveforms of FIG. 12A are formed in a singletiming channel as discussed earlier.

The waveforms of FIG. 12A can be beneficial, because a small non-zeroreturn current (I2) during gaps 172 can enhance polarization of neuralfibers. The combination of the polarizing return current (I2) withsuperimposed higher-intensity sub-phase pulses 171 a and 171 b (I1) cancreate changes in excitability and recruitment order. For example, largefibers can be excited once, such as during a first sub-phase pulse 171 a1 in first pulse phase 170 a, while smaller fibers are excited multipletimes by all of the sub-phase pulses 171 a in the first pulse phase 170a, or at least during subsequent sub-phase pulses 171 a 2.

FIG. 12A also shows how the GUI 120 (FIG. 8) can be modified to form thewaveforms of FIG. 12A. Specifically, the stimulation parametersinterface 122′ of GUI 120′ has been changed to add for the setting oradjustment of the two current levels I1 and I2 used to define thepulses. Settings to adjust the low (f_(L)) and high (f_(H)) frequencyaspects of the pulses, and pulse widths PW_(L) and PW_(H), can beincluded as before, as can other aspects not shown in FIG. 12A.

FIG. 12B shows a modification to the waveforms of FIG. 12A. In thisexample, the non-zero return current (I2) is established at thebeginning and end of each of the pulses phases 170 a and 170 b. That is,the pulse phases 170 a and 170 b start and end with the non-zero returncurrent, rather than starting and ending with the sub-phase pulses 171 aand 171 b (I1), as occurred in FIG. 12A. Particularly at the start ofthe phases 170 a and 170 b, providing the non-zero return current beforethe sub-phase pulses can assist in neural polarization, and henceselective recruitment of different neural fibers as described earlier.

FIGS. 13A-13F show other examples of stimulation waveforms having bothlow frequency (f_(L)) and high frequency (f_(H)) features, and similarto FIGS. 12A and 12B in that the current does not return to zero in gaps182 between the sub-phase pulses 181 a and 181 b. Again, the waveform atonly a single selected electrode (e.g., E1) is shown.

In FIG. 13A, the current amplitudes of the sub-phase pulses 181 a and181 b, the non-zero return current during gaps 182, or both, are notconstant over the duration of the first and second pulse phases 180 aand 180 b, but instead vary. Thus, during the first pulse phase 180 a,the amplitude of the sub-phase pulses 181 a varies as value +I1, whilethe amplitude of return current varies as value +I2, both showngenerally with dotted lines. During the second pulse phase 180 b, theamplitude of the sub-phase pulses 181 b and the non-zero return currentvary as values −I1 and −I2. Nonetheless, the two phases 180 a and 180 bare preferably charge balanced at each electrode, i.e., with the chargeprovided by the sub-phase pulses 181 a and the non-zero return currentequaling +Q during the first pulse phase 180 a and the charge providedby the sub-phase pulses 181 b and the non-zero return current equaling−Q during the second pulse phase 180 b.

FIG. 13B shows another example in which only the amplitude of thenon-zero return current (I2) during gaps 182 varies, and specifically isramped; the amplitude of the sub-phase pulses 181 a and 181 b are heldconstant (I1). The variation in the amplitudes in FIGS. 13A and 13B areshown to vary similarly during phases 180 a and 180 b, but as beforethis isn't required. For example, in FIG. 13C, the amplitude of thenon-zero return current and the sub-phase pulses 181 a are constantduring the first pulse phase 180 a; however, the amplitude of thenon-zero return current varies during the second pulse phases 180 b. Theamplitude of the sub-phase pulses 181 b could also vary during thesecond pulse phase 180 b, although this isn't shown in FIG. 13C. Even ifthe pulse phases 180 a and 180 b are not symmetrical as shown in FIG.13C, they can still be charge balanced (+Q and −Q).

In FIG. 13D, the current amplitudes of the sub-pulses 181 a and 181 b(+I1 or −I1) and the return currents (+I2 and −I2) are constant over theduration of the first and second pulse phases 180 a and 180 b, but thehigh-frequency pulse widths PW_(H) of the sub-phase pulses 181 a and 181b vary during each of the pulse phases. The variation in the pulse widthPW_(H) is again shown to vary similarly during phases 180 a and 180 b,but this isn't required. Again, the two phases 180 a and 180 b arecharge balanced at each electrode (+Q=|−Q|).

In FIG. 13E, charge balancing at each electrode is provided even thoughsub-phase pulses are provided during only one of the pulse phases 180 aor 180 b. For example, in FIG. 13E, only first pulse phase 180 a hasmonophasic sub-phase pulses 181 a and a non-zero return current (oftotal charge +Q); the second pulse phase 180 b comprises a constantpulse 112 a or 112 b (of charge −Q). The duration or amplitude of theconstant pulse 112 a or 112 b can vary. For example, constant pulse 112a has a pulse width equal to the duration of the first pulse phase 180a, but an amplitude −I3 that is smaller than the amplitude +I1 of thesub-phase pulses 181 a and larger than the amplitude +I2 of the non-zeroreturn current. Constant pulse 112 b has an amplitude Al equal to theamplitude +I1 of the sub-phase pulses 181 a, but has a pulse width thatis smaller than the duration of the first pulse phase 180 a. In eithercase, the charged is balanced during both pulses phases 180 a and 180 b(+Q=|−Q|).

FIG. 13F shows a different example of waveforms in which the non-zeroreturn current is equal and of the same polarity in both of the pulsephases 180 a and 180 b. In the first pulse phase 180 a, sub-phase pulses181 a are provided having a positive amplitude of +I1. However, in thegaps 182 between the sub-phase pulses 181 a, the non-zero return currentis negative, having an amplitude of −I2. In total, the first pulse phase180 a can have a net charge of +Q, with the positive sub-phase pulses181 a adding to this net value and the negative return currentsubtracting from this net value. In the second pulse phase 180 b, thesub-phase pulses 181 b have a negative amplitude of −I3. In the gaps 182between the sub-phase pulses 181 b, the non-zero return current has thesame negative amplitude of −I2 that occurred in the first pulse phases180 a. In total, the second pulse phase 180 b can have a net charge of−Q, with the negative sub-phase pulses 181 b and the negative returncurrent contributing to this net charge. To achieve charge balancebetween the first and second pulses phases 180 a and 180 b (+Q and −Q),the absolute value of −I3 would be smaller than +I1. The waveform ofFIG. 13F can be beneficial because it provides the same degree of neuralpolarization during the gaps 182 in both of the pulse phases 180 a and180 b.

The waveforms illustrated can also be used with passive charge recovery.This is shown in FIGS. 14A and 14B for a waveform having a zero returncurrent during gaps 192 (FIG. 14A), and a waveform having a non-zeroreturn current during gaps 192 (FIG. 14B). In these examples, sub-phasepulses 191 a and the non-zero return current (FIG. 14B only) areactively driven by the stimulation circuitry 28 (FIG. 3) only during afirst pulse phase 190 a. There is no actively-driven second pulse phase.Instead, a passive recovery phase 190 c is provided after the firstpulse phase 190 a. As explained earlier, passive recovery can recovercharge stored on capacitive elements in the current path (e.g., betweenelectrodes E1 and E2), such as the DC-blocking capacitors 38. Thisoccurs using passive recovery switches 41 _(i), as shown in the circuitdiagram of FIG. 14C. After the first pulse phase 190 a, capacitors C1and C2 38 coupled to the electrodes E1 and E2 would be charged (Vc1,Vc2) with the polarities as shown. When the passive recovery switches411 and 412 connected to electrode nodes e1 and e2 39 are closed duringthe passive recovery phase 190 c, the electrode nodes are shorted to areference voltage (e.g., Vbat). This causes the charge on the capacitorsto equilibrate, causing a current 193 flow from E2 to E1 through thepatient's tissue, R. Given the R-C nature of this circuit, this current193 will exponentially decay, and assuming the passive recovery switches41 _(i) are closed for a long enough duration, the voltages across thecapacitors and the resulting current 193 will decay to zero. Thus, thecharge of the first pulse phases (+Q) is passively recovered during thepassive recovery phase 190 c (−Q).

To this point, the waveforms have been shown as having pulses with onlytwo phases, such as the first and second pulse phases 106 a and 106 b ofFIG. 7, the first and second pulse phases 170 a and 170 b of FIG. 12A,or the first and passive recovery pulse phases 190 a and 190 b of FIGS.14A and 14B. However, each pulse could have more than two pulse phases,and all pulse phases can be charged balanced with a pulse, although thisis not illustrated for simplicity.

The modifications to the various waveforms illustrated to this point canbe combined in different manners, even if such combinations are notillustrated in the figures. It is not practical to illustrate all ofthese possible combinations, but it should be understood that anycombination of the various modifications can be used in a practicalimplementation and are within the scope of this disclosure.

Although particular embodiments of the present invention have been shownand described, it should be understood that the above discussion is notintended to limit the present invention to these embodiments. It will beobvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present invention. Thus, the present invention is intended to coveralternatives, modifications, and equivalents that may fall within thespirit and scope of the present invention as defined by the claims.

What is claimed is:
 1. A stimulator device, comprising: a plurality ofelectrode nodes, each electrode node configured to be coupled to one ofa plurality of electrodes configured to contact a patient's tissue; andstimulation circuitry configured to provide in a single timing channel asequence of pulses at at least two of the electrode nodes selected tocreate a stimulation current through the patient's tissue, wherein thestimulation circuitry is configured to form each pulse at the selectedelectrode nodes with a first phase and a second phase, wherein the firstphase at each selected electrode node comprises a plurality of firstmonophasic sub-phase pulses of a first polarity and separated by gapsduring which no current is issued to the tissue, wherein the secondphase at each selected electrode node comprises a plurality of secondmonophasic sub-phase pulses of a second polarity opposite the firstpolarity and separated by gaps during which no current is issued to thetissue, and wherein at each selected electrode node a first total chargeof the plurality of first monophasic sub-phase pulses is equal butopposite a second total charge of the plurality of second monophasicsub-phase pulses, wherein each first and second monophasic sub-phasepulse is of a constant amplitude.
 2. The stimulator device of claim 1,further comprising a case for housing the stimulation circuitry, whereinthe case is conductive, and wherein the conductive case comprises one ofthe plurality of electrodes.
 3. The stimulator device of claim 1,wherein at least one selected electrode node is coupled to itsassociated electrode through a DC-blocking capacitor.
 4. The stimulatordevice of claim 1, further comprising at least one implantable lead,wherein the electrodes are located on the lead.
 5. The stimulator deviceof claim 1, wherein the constant amplitude comprises a constant current.6. The stimulator device of claim 1, wherein a pulse width of the firstmonophasic sub-phase pulses varies during the first pulse phase, orwherein a pulse width of the second monophasic sub-phase pulses variesduring the second pulse phase, or wherein the pulse width of the firstmonophasic sub-phase pulses varies during the first pulse phase and thepulse width of the second monophasic sub-phase pulses varies during thesecond pulse phase.
 7. The stimulator device of claim 1, wherein afrequency of the first monophasic sub-phase pulses varies during thefirst pulse phase, or wherein a frequency of the second monophasicsub-phase pulses varies during the second pulse phase, or wherein thefrequency of the first monophasic sub-phase pulses varies during thefirst pulse phase and the frequency of the second monophasic sub-phasepulses varies during the second pulse phase.
 8. The stimulator device ofclaim 1, further comprising control circuitry, wherein the controlcircuitry is configured to receive a plurality of stimulation parametersincluding a first frequency of the pulses, a second frequency of thefirst and second monophasic sub-phase pulses, a pulse width of at leastone of the first and second phases, and a pulse width of the first andsecond monophasic pulses, wherein the control circuitry is configured touse the stimulation parameters to provide a plurality of control signalsto the stimulation circuitry to cause the stimulation circuitry to formthe sequence of pulses at the selected electrode nodes.
 9. Thestimulator device of claim 8, further comprising an antenna, wherein thecontrol circuitry is configured to receive the stimulation parametersfrom the antenna.
 10. The stimulator device of claim 8, wherein thecontrol circuitry is configured to produce a first digital signal at thesecond frequency, a second digital signal at the first frequency andcorresponding to a timing of the first phase, and a third digital signalat the first frequency and corresponding to a timing of the secondphase.
 11. The stimulator device of claim 10, wherein the stimulationcircuitry comprise a plurality of switches, wherein the plurality ofswitches are controlled by a mixture of the first and second digitalsignals during the first phase, and wherein the plurality of switchesare controlled by a mixture of the first and third digital signalsduring the second phase.
 12. The stimulator device of claim 1, whereinthe stimulator device comprises an implantable pulse generator.
 13. Thestimulator device of claim 1, wherein the stimulator device comprises anexternal stimulator.
 14. The stimulator device of claim 1, wherein thestimulation circuitry is configured to form an interphase period at theselected electrode nodes between the first phase and the second phase,wherein no current is issued to the tissue during the interphase period.15. The stimulator device of claim 1, wherein the first monophasicsub-phase pulses are positive at at least one of the selected electrodenodes and negative at at least one other of the selected electrode nodessuch that the net current injected into the tissue at any time is zeroduring the first phase, and wherein the second monophasic sub-phasepulses are negative at the at least one of the selected electrode nodesand positive at the at least one other of the selected electrode nodessuch that the net current injected into the tissue at any time is zeroduring the second phase.